1. Field of the Invention
The present invention relates to an active filter and a portable telephone apparatus. More particularly, the present invention relates to an active filter suitable for an IC and a portable telephone apparatus using this filter.
2. Background of the Invention
Radio telephones or portable telephones, for example, analog cellular telephones in an E-TACS (Extended-Total Access Communication System), are allocated the following frequencies:
______________________________________ Downlink channel frequency: 917.0125-950.0125 MHz Channel frequency: 12.5 kHz ______________________________________
For this reason, the signal receiving circuit used in E-TACS type cellular telephones may, for example, be a double super heterodyne circuit as shown in FIG. 1. In FIG. 1, a downlink channel FM signal S1 (frequency f1) from a base station is received by an antenna 1. The signal S1 received by the antenna 1 is supplied, through a radio frequency amplifier 2 and a band pass filter 3 which has a bandwidth passing through all the downlink channels, to a first mixer circuit 4. An oscillating signal S11 of a frequency f11 corresponding to a channel used for communication is output from a PLL (Phase Locked Loop) circuit 11. The oscillating signal S11 from the PLL circuit 11 as a first local oscillating signal is supplied to the first mixer circuit 4. In the first mixer circuit 4, the received signal S1 is converted into a first intermediate frequency signal S4 having a frequency f4 given by the following: ##EQU1##
This first intermediate frequency signal S4 is supplied to a second mixer circuit 6 through a first intermediate frequency circuit 5, and an oscillating signal S12 having a frequency f12 given by the following: EQU f12=54.95 MHz
is output from a PLL circuit 12. This oscillating signal S12 as a second local oscillating signal is supplied to the second mixer circuit 6.
In the second mixer circuit 6, the first intermediate frequency signal S4 is converted into a second intermediate frequency signal S6 having a frequency f6 given by the following: ##EQU2##
This second intermediate frequency signal S6 is supplied to an FM demodulating circuit 8 through a second intermediate frequency circuit 7, then an audio signal according to a voice of a user as a called party or a data signal from a base station, etc., are output from a terminal 9.
The above mentioned structures and operations of the receive circuit are general structures or operations in an analog cellular telephone.
However, as shown in FIG. 1, the second intermediate frequency circuit 7 has a band pass filter 7A that passes the second intermediate frequency signal S6 and an amplifier 7B that amplifies the second intermediate frequency signal S6. The first intermediate frequency circuit 5 has the same structure as the second intermediate frequency circuit 7. In general, the band pass filter 7A is composed of a ceramic filter device.
When the band pass filter 7A is composed of a ceramic filter device, the second intermediate frequency circuit 7, a former part of the circuit 7 and a later part of the circuit 7 cannot be integrated as an IC even if it is desired to do so. Consequently, when fabricating an IC circuit, the ceramic filter device must be mounted on a separate circuit board apart from the IC, and as a result, the advantage of the IC is less.
It has therefore been proposed to constitute the band pass filter 7A as an active filter so that it is possible to fabricate an IC.
However, in this case, as an analog cellular portable telephone is used in a narrow bandwidth FM system, the deviation of the center frequency of the band pass filter 7A is permitted only to be on the order of .+-.2 kHz. At this time, the second intermediate frequency f6 is 50 kHz. In other words, the deviation of the center frequency of the band pass filter 7A is therefore permitted only to be about .+-.4%.
However, if the active filter is composed of an IC, it is difficult to maintain the absolute values of resistances of resistors and electrostatic capacities of capacitors within a range of .+-.4% error values, and moreover, it is necessary to deal with variations in these values due to temperature characteristics. Hence, when the band pass filter 7A is integrated in an IC to use an active filter, it is necessary to be able to adjust or control the center frequency of the band pass filter 7A.
For example, when constructing the band pass filter 7A using an active filter circuit, it may include the low pass filter part shown in FIG. 2.
According to this construction, the second intermediate frequency signal S6 from the second mixer circuit 6 is supplied to a differential amplifier A1 having transistors Q1, Q2 and emitter resistors R1, R2, and is voltage/current converted. The signal S6 thereby converted to a current is then supplied to a differential amplifier A2 having transistors Q3, Q4, diodes D1, D2 and a constant current source Q5 connected to the input portion of this differential amplifier A2. The output of the differential amplifier A2 is supplied to an operational amplifier A3. An integrating capacitor C1 is connected to the operational amplifier A3.
In the circuit shown in FIG. 2, a low pass filter 7C is composed of the output impedance of the amplifier A2 and the input capacitance of the operational amplifier A3. As a result, the operational amplifier A3 outputs the signal S6 in which the high frequency components thereof are eliminated. If a high pass filter is connected to the output portion of the operational amplifier A3, the band pass filter 7A is able to be constituted.
In other words: EQU fc=1/(2.pi.C1/gm)
wherein
gm=mutual conductance of amplifier A2, and PA1 fc=cutoff frequency of low pass filter 7C. PA1 I1=constant current of constant current source Q5, and PA1 I2=constant current of constant current source Q6 of amplifier A2.
In this case, EQU gm=1/(2R1).times.(I2/I1)
wherein
The high frequency components of the signal input to the operational amplifier A3 are eliminated by the low pass filter 7C, and if the magnitude of I1 or I2 is controlled, the cutoff frequency fc of the low pass filter 7C may be adjusted or controlled.
However, in the low pass filter 7C shown in FIG. 2, the transistors Q1, Q2 and the transistors Q3, Q4, are arranged in a DC voltage cascade connection. The source voltage VCC of these transistors must be set high to obtain sufficient dynamic range.
The noise signals generated by the diodes D1, D2 and the base resistances of the transistors Q3, Q4 are amplified by the differential amplifier A2 and are mixed with the second intermediate frequency signal S6, so the noise level is at a high level. As a result, equivalent receiver sensitivity of the telephone decreases, hence this low pass filter 7C is not suitable as an intermediate frequency filter.